The welding method normally used in connection with fastening of strips of this kind onto separation discs is spot welding. This welding method is cheap and has been considered sufficient from different starting points. Thus, brazing has long been abandoned as a connection method, and welding of the strips along all of the edges thereof has been considered unnecessarily expensive and not required.
However, in certain connections new requirements have lately been set up on the connection of the strips with the separation discs, meaning that the edges of each strip have to be sealingly connected with its separation disc, so that no pockets are formed, which are difficult to clean.
These new requirements have brought up the question how, in the cheapest and most practical way, a continuous weld joint can be provided through resistance welding between a thin strip of sheet metal of this kind and a conical separation disc also produced from thin sheet metal so that heat, developed during the welding, will not cause deformation of the separation disc. A conical separation disc is usually produced by pressure turning of a circular or annular planar piece of thin sheet metal and it obtains, as a consequence of this treatment, stresses built into the sheet material, which may easily deform the separation disc upon local heating thereof.
A problem in connection with use of conventional roller welding for obtaining a continuous weld joint between a strip and a separation disc is that a relatively large contact surface comes up between the strip and the separation disc, when a roll welding electrode is rolled along the strip and presses it against the separation disc. This contact surface comes up in a welding area extending across the whole of the strip and over a distance in the longitudinal direction of the strip. In order to obtain melting in this relatively large contact area relatively much energy has to be used, which leads to an undesired strong heating of the separation disc.
Theoretically it would be possible to limit the welding to only a small part of the width of the strip for minimizing the heat development during a welding operation. However, this would require that both the weld electrode applied and pressed against the strip and the weld electrode applied against the separation disc were isolated electrically over certain parts of their surfaces intended for contact with the strip and the separation disc, respectively, and further that the uninsulated area or areas of the respective contact surfaces of the weld electrodes were to be placed extremely accurately in relation to both each other and the strip all the way along where a weld joint should be obtained. Considering that the strips here in question have a width of about 3-6 mm, that a weld joint of maximum 1 mm width is deemed desirable, that the strip is to be applied onto a curved (conical) surface and that the strip (weld joint) should not always be straight and extend along, but sometimes at an angle, to a generatrix of the separation disc, an apparatus for obtaining this would become very complicated and expensive.
For resolving the problem of undesired local overheating of a conical separation disc, when a narrow strip of the kind here in question is to be connected by resistance welding with a separation disc through two parallel weld joints along the edges of the strip, it has been suggested in EP 0 444 271-A3 that a weld electrode in the form of a relatively narrow roller should be rolled across the strip in its cross direction and that welding current should be switched on only when the roller passes over the edges of the strip, where a weld joint should be obtained, and be switched off while the roller rolls across the central part of the strip between the edges. Thus, the roller has to be rolled across the strip several times before two continuous weld joints have been obtained along the whole of the strip. It is also suggested in EP 0 444 271-A3 that the weld roller should be rolled along the entire circumference of the separation disc and that welding should thus occur only when the roller passes across the edges of the strips in question, which in the case disclosed are placed such that they extend along the generatrices of the conical separation disc.
From various starting points it may be questioned whether the welding method suggested in EP 0 444 271-A3 is really performable without obtaining of an undesired local overheating of the separation disc. If it would be possible, after all, the weld roller would probably have to be very narrow, which makes it apparent that a welding operation performed in this manner would take far too long to be useful in practice.